In a conventional method, a mobile station of an orthogonal frequency division multiplexing (OFDM) system uses a preamble signal, which is positioned in a head of an OFDM frame, as a reference signal, measures a power value of the received preamble signal, and controls a gain of an analog amplifier (AMP) according to a difference between the measured power value of the preamble signal and a threshold value.
In the conventional method, there is no problem in estimating a given environment by using a preamble signal, establishing a gain of an amplifier (AMP) of an automatic gain device, and applying the established gain to a data symbol section, when a length of a frame is short and a moving speed of the mobile station is limited. However, many problems occur in applying a channel environment measured during a preamble section to the subsequent data symbol section when the length of the frame increases and the mobile station moves at a high speed.
FIG. 1 is a schematic diagram of a configuration of an automatic gain controller of a mobile station of a conventional OFDM system. FIG. 1 shows a receiving unit of a conventional mobile station of an IEEE. 802.11 OFDM system. A frame signal (an analog signal) that is received from the mobile station through a radio frequency (RF) unit 101 is amplified/attenuated by an amplifier 110 of an automatic gain controller 102 to be a signal that is required by an analog/digital converter (ADC) 103, and is converted to digital data by the ADC 103. In the OFDM system, the converted digital data is converted to frequency domain data by a fast Fourier transform (FFT) unit 104.
In addition, during a preamble section, a channel estimator 105 measures received power of a preamble to compensate signal fading caused in a radio propagation, and a gain controller 111 of the automatic gain controller 102 compares a predetermined threshold value and the measured power value of the preamble to establish a gain of the amplifier 110 of the automatic gain controller 102 according to a difference between the threshold value and the measured power value to operate the automatic gain controller 102.
During a subsequent data symbol section, the same gain is maintained, which is applied to drive the automatic gain controller in the period of frames.
FIG. 2 shows a diagram of a frame configuration of a conventional OFDM-time division duplexing (TDD) system. A preamble is provided to a head of the frame to perform initial synchronization, cell search, and channel estimation. Subsequent DL and UL data symbols include a data subcarrier to allocate data as shown in FIG. 8 and a pilot subcarrier to transmit pilot information for estimating a channel.
During a data symbol section, a channel estimator 105 measures phase, amplitude, and power of a pilot signal in the received data symbol as shown in FIG. 3, measures frequency fading according to a difference between the measured values of the pilot signal and values of a reference pilot signal, and establishes a parameter of an equalizer to compensate a phase error and an amplitude error. Data converted to be in a frequency domain is transmitted to the equalizer 106 to compensate the frequency fading. The data having the compensated frequency fading generally perform an actual demodulation process in a decoder 107 in the OFDM system.
FIG. 3 shows a graph representing a reference signal and a received signal of a pilot channel in a symbol pilot channel estimator. P1 denotes the predetermined pilot signal (i.e., the reference signal), and P2 denotes the received signal. The symbol pilot channel estimator 109 calculates a phase error and a power error according to a difference between P1 (reference signal) and P2 (received signal), and then establishes a parameter to compensate the frequency fading. In the above method, signal fading caused in the radio propagation is calculated for symbol pilot carrier information, a channel compensation value in a symbol frequency bandwidth is estimated by using an interpolator, and the parameter of the equalizer 106 is established. The data converted to be in the frequency domain by the FFT unit 104 is transmitted to the equalizer 106 to compensate the frequency fading. The compensated data is demodulated and decoded by the decoder 107 in the OFDM system.
In the automatic gain control method of the mobile station of the conventional OFDM system, there is no problem in estimating a radio propagation channel by using the preamble signal, and establishing the gain of the amplifier 110 of the automatic gain controller 111 when a length of the frame is short and a moving speed of the mobile station is limited. However, many problems occur in applying the channel environment measured during the preamble section to the subsequent data symbol section when the length of the frame increases and the mobile station moves at a high speed.